“In-mold molding” is a technique used, when injection molding of plastic, ceramic or the like is carried out, to transfer an image or photograph from a film with a design on it placed between mold halves onto a surface of the plastic or the like in the mold simultaneously with the injection molding. A “transfer film for in-mold molding” is a printing film used in an in-mold molding technique in which a transfer layer is peeled off a substrate film and transferred to a molded article as a target of transfer to decorate it or otherwise impart an additional function to it. Transfer films for in-mold molding are widely used to decorate or protect the surface of, for example, the housings of cellular phone terminals, notebook PCs and digital cameras, home electric appliances and cosmetic containers, and automotive parts.
The simultaneous molding and decoration process in which a film is set in the mold for injection molding is divided into two types: IMD (In-Mold Decoration), in which the film (residual film) does not remain on the molded article, and IML (In-Mold Lamination), in which the film remains on the molded article.
Of the two processes, the IMD (In-Mold Decoration) does not require cumbersome steps, such as preforming and trimming, in contrast to IML because the residual film is removed from the molded article after the molding and transfer, and therefore can allow automation and speed-up of the decorating and molding steps. In addition, IMD has a significant effect on improvement of productivity and cost cutting and therefore demonstrates a significant advantage in the production of mass-produced items where economies of scale are required.
A transfer film for in-mold molding is typically composed of a substrate film, a mold release layer, an IMD layer, a print layer and an adhesion layer, and is peeled off at the interface between the mold release layer and the IMD layer after injection molding.
Thus, the molded article has the IMD layer as the outermost layer, and the role of the IMD layer is very important to obtain a molded article excellent in durability, chemical resistance and moldability.
As a method for obtaining a molded article excellent in durability and chemical resistance, a layer composed of an active energy beam-curable resin has been conventionally used as the IMD layer.
One method for the production of a transfer film for in-mold molding is a method in which the film is irradiated with an active energy beam during its production to crosslink and cure (precure) the active energy beam-curable resin. In this method, however, the ability of the IMD layer to conform to the molded article during molding is so poor that the IMD layer tends to develop cracks easily. In order to prevent the occurrence of the cracks, a method is widely adopted in which the film is not irradiated with an active energy beam during its production and the IMD layer as the outermost layer after the transfer onto the molded article is irradiated with an active energy beam to crosslink and cure (aftercare) the active energy beam-curable resin. This method, however, has many problems as described below.
When a film is produced, the active energy beam-curable resin remains runny and sticky when subjected only to a heat-drying process. Thus, problems, such as transfer of the resin to the guide roll and blocking on the reel, occur during a roll-to-roll production process.
In addition, when the resin to be molded is injected into the mold for injection molding, a phenomenon in which the IMD layer or print layer in the vicinity of the injection port (gate) of the mold is caused to flow out by the injected resin (gate flow) can occur.